1. Field of the Invention
The present invention relates to a diaphragm control device for cameras, which determines the shooting aperture of an exchangeable lens by stopping the operation of a diaphragm actuating mechanism, which causes the diaphragm control member of the exchangeable lens to stop down the diaphragm of the exchangeable lens, when an electric signal representing the amount of operation of the diaphragm actuating mechanism coincides with a preset reference value.
2. Description of the Prior Art
Most of the known diaphragm control devices of the above described type employ an electromagnetic mechanism including an electromagnet for stopping the operation of the diaphragm actuating mechanism provided on a camera body. A diaphragm stop signal to invert the state of the electromagnet upon the coincidence of the electric signal representing the amount of operation of the diaphragm actuating mechanism with a preset reference value is generated by an aperture control circuit. The electromagnetic mechanism stops the diaphragm actuating mechanism mechanically in response to the inversion of the state of the electromagnet. However, there is a certain time lag from the generation of the diaphragm stop signal till the stop of the operation of the diaphragm actuating mechanism by the electromagnetic mechanism, so that the actual diaphragm aperture thus determined will be smaller than an optimum diaphragm aperture if the diaphragm stop signal is generated when the diaphragm actuating mechanism has operated to a position providing an optimum diaphragm aperture, and hence accurate aperture control is impossible. To cope with this problem, a method of advancing the time of generation of the diaphragm stop signal by a fixed time corresponding to the delay of the actuation of the electromagnetic mechanism has been employed. The operating speed of the diaphragm actuating mechanism must be stable over the entire diaphragm control range to always achieve accurate aperture control by this known method. However, since the diaphragm actuating mechanism is accelerated gradually when the diaphragm actuating mechanism is operated by a spring, which is an ordinary measure to operate a conventional diaphragm actuating mechanism, it has been difficult to stabilize the operating speed of the diaphragm actuating mechanism, and accordingly accurate diaphragm control cannot be always expected even by the above method. Especially in the case of diaphragm control devices disclosed in Japanese Patent Laid-open Publication No. 53-86209 and U.S. Pat. No. 4,396,272, the respective starting positions of the diaphragm control member of a diaphragm actuating mechanism and a diaphragm control member of an exchangeable lens interlocked with the former diaphragm control member are defined by stopper means mounted on a camera body or on the exchangeable lens, and the amount of the displacements of the diaphragm control members from the respective starting positions are converted into pulse signals. The exchangeable lens used in the diaphragm control devices need not be provided with a mechanism accepting overloading of the diaphragm control members because their starting positions are defined by the stopper means and hence the exchangeable lens can be constructed in a compact structure. However, since the diaphragm control members are accelerated immediately after the start of the stop-down operation thereof, it is difficult to accurately determine the diaphragm aperture particularly in the vicinity of the starting positions of the diaphragm control members, that is, in the vicinity of a fully open aperture value.